lunes, 1 de abril de 2013

CO2, but from where?

Geologists have many records that tell the story of the last glacial maximum, the time between about 20,000 and 15,000 years ago when the glaciers of the last ice age reached their peak size and started to retreat. Ice cores, sediment cores, records of plants, soil, wind-blown loess deposits, ice-rafted debris in the ocean, etc. One story told over and over is that CO2 in the atmosphere went up significantly, from about 180 ppm to 280 ppm (for comparison, we’re currently very close to 400 ppm). That CO2 pulse into the atmosphere warmed the planet and created a runaway process that melted the glaciers.

One big question has always remained though; where did this CO2 come from? We know where the CO2 pulse today is coming from; fossil fuels, but 15,000 years ago there were no coal plants.

There are only a couple sources that could supply such a burst of carbon to the atmosphere other than fossil fuels. If most of the biosphere was killed, that would do it, but that didn’t happen. The release happened over a few thousand years, so it must have been an ongoing process; it couldn’t have been cataclysms like a meteor impact or anything else remarkable.

That pretty much leaves one source of carbon, the ocean. The ocean holds a huge amount of carbon, around 50 times as much as the atmosphere. Releasing a portion of that to the atmosphere could account for the burst, but that leaves a pair of huge questions: how did it happen, and could it happen again?

To make this carbon-burst happen, the Earth somehow needs to bring carbon-rich waters from the deep ocean up to the surface. The waters at the bottom of the ocean are there because of density; the waters form in the Antarctic and the North Atlantic, where they gain density because they are cold and because the waters are somewhat salty (temperature is more important today).

Most proposals for overturning the ocean involve winds. If the location of strong winds change on the globe that could change the mixing of the ocean and alter bottom water formation, but this is difficult to prove since winds don’t leave clear records for us to interpret.

This week’s issue of Nature gives a new hypothesis, and conveniently, it can be tested in the geologic record. A group of geoscientists led by ETH in Zurich realized that as the deglaciation started, sediments throughout the mid-Atlantic region suddenly had a large pulse of opal.

Opal is a silicate phase that in the oceans commonly forms from the leftovers of diatoms. You can see some diatoms in this image; they are planktonic life forms which float in the ocean and make small (and quite lovely) shells out of silica. When they die, those shells sink, eventually becoming opal layers.

An opal layer at this horizon means that something introduced more silica to the ocean as the glaciers started to break up. That was their hint, the North Atlantic Deep Water. Waters from somewhere else must have replaced this water as the glaciers broke apart; somehow the North Atlantic deep water must have weakened or gone away.

When glaciers like the ones in Greenland today melt, they release a burst of fresh water into the ocean. That fresh water can be enough to overcome the density difference causing North Atlantic water to sink. As the glaciers receded, the freshwater pulse weakened the formation of deep water, allowing some other water to take its place. (This mechanism, of course, has been suggested previously, to the point of being featured in the highly exaggerated movie “The Day after Tomorrow”).

That other water was silica rich, coming from the continental shelves and from Antarctica. Without this North Atlantic Deep Water formation, the silica rich water penetrated everywhere into the ocean, leading to the surge of diatom growth and opal formation. When this happens, it overturns part of the ocean, allowing CO2 into the atmosphere.

That’s a mechanism for ventilating CO2 from the ocean which not only works, but also leaves a geologic record! It’s difficult to look for records of wind; it’s easy to look for sediment pulses, and in multiple places throughout the Atlantic, that opal record exists. In fact, going back through 5 different deglaciations, there is an opal layer at each one, suggesting that mixing of high-silica water into the full Atlantic is a key part of every deglaciation we have records for.

This may not be the only part of the story for the CO2 burst; this process cannot start the CO2 rise, only amplify it. But, the presence of this silica layer is strong evidence that the normal, silica-poor deep waters went missing, and those waters are generated in the North Atlantic.

That mechanism has relevance for today as well, since there is a large glacial body still sitting in the North Atlantic, and since there is a lot of CO2 currently locked in the ocean. If weakening the formation of North Atlantic Deep Water leads to increased ocean ventilation, then meltwater from the Greenland ice sheet could be enough to cause additional release of oceanic CO2, which is scary in terms of runaway warming processes.